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Version History of Internet Explorer 9.0 Beta (7 x64)

User interface

IE9 includes signficant alterations to its user interface when compared with previous versions. These include integration with Windows 7's taskbar providing Pinned Sites, a new security-enabled download manager that manages file transfers and can pause and resume downloads and informs if a file may be malicious, address bar search, enhanced tabs and tab page, and an Add-on Performance Advisor that shows which third party add-ons may be slowing down browser performance and then allows the option to disable or remove them. It introduces a minimalist interface, which brings it more in line with the current interfaces of Chrome and Opera. It is also reminicent of the Windows Live suite.

JScript engine

IE9 features a faster JScript engine than IE8's, internally known as Chakra. Chakra has a separate background thread for compiling JavaScript. Windows runs that thread in parallel on a separate core when one is available. Compiling in the background enables users to keep interacting with webpages while IE9 generates even faster code. By running separately in the background, this process can take advantage of modern multi-core machines. In preliminary SunSpider benchmarks for the third IE9 Platform Preview, it outperformed the IE8 engine by a factor 10 and also outperformed the newest Firefox 4.0 pre-release.

ECMAScript
The Chakra JScript engine significantly improves support for ECMA-262: ECMAScript Language Specification standard, including features new to the recently finalized Fifth Edition of ECMA-262 (often abbreviated ES5).

DOM

DOM improvements include:

DOM Traversal and Range

Full DOM L2 and L3 events

getComputedStyle from DOM Style

DOMContentLoaded

CSS

CSS improvements include support for portions of the following modules:

CSS3 Backgrounds and Borders

CSS3 Values and Units

CSS3 Media Queries

HTML5

HTML5 Media

IE9 includes support for the HTML5 video and audio tags.
The audio tag will include native support for the MP3 and AAC codecs, while the video tag will natively support H.264/AVC. Support for other video formats, such as WebM, will require third-party plugins.

SVG elements that are supported in the Platform Preview are fully implemented. Elements that exist in the Platform Preview have corresponding SVGDOM support and can be styled with CSS/presentation attributes.
The final build of IE9 will also support:

Methods of embedding: <embed>, <iframe>, <img>, css image, .svgz

Gradients and Patterns

Clipping, Masking, and Compositing

Cursor, Marker

Remainder of Text, Transforms, Events

Web typography

Internet Explorer was the first browser to support web fonts through the @font-face rule, but only supported the Embedded Open Type (EOT) format, and lacked support for parts of the CSS3 fonts module. IE9 completed support for the CSS3 fonts module and added WOFF support.

User agent string

Due to the technical improvements, the IE developer team decided to change the user agent string. The Mozilla/4.0 token was changed to Mozilla/5.0 to match the user agent strings of other browsers and to indicate that IE9 is more interoperable than previous versions. The Trident/4.0 token was likewise changed to Trident/5.0. Unlike previous versions, IE9 does not send any of the .NET identifiers or other "post platform tokens" to servers as part of the user agent string, except if running in Compatibility View (to render as IE7 did - in such a case, the same post platform tokens that were sent with those browsers are sent).